May 12, 1999

Star Wars by Moonlight

Venus and the Moon will put on a dazzling show for moviegoers May 17 through May 19

Credit & Copyright: S. Barnes, Sky Optics
May 12, 1999: Next week thousands of science and science fiction fans may find themselves standing in long lines, waiting to buy tickets for the newest episode of Star Wars. If you're one of them, don't forget to look up. Venus and the Moon are putting on a sky show that may help pass the time.

Right: The crescent Moon, Venus, and Jupiter all appeared together in the early morning hours of April 23rd, 1998. A similar show -- minus Jupiter -- will take place on May 17th through 19th, 1999. This image appears courtesy of S. Barnes, Sky Optics, copyright 1998 all rights reserved.

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During the month of May Venus is high above the evening horizon and will be visible for over 3 hours after sunset. The second planet from the sun is impossible to miss, even if you're surrounded by city lights. It blazes at magnitude -4.2, second only to the Sun and the Moon in brightness. To find it just after sunset, face southwest and look approximately 40 degrees above the horizon. When Venus is this bright it is frequently mistaken for an airplane, but if you wait a few moments and it doesn't move then you've found Venus.

This original art
 appears with the permission
of Bishop Web Works, copyright 1999
Venus, which reaches its greatest elongation from the Sun on June 11, is spectacular throughout the month of May. Between May 17th and 19th the slender crescent Moon will join Venus in the southwestern sky for a memorable show.

On Monday the 17th, about an hour after sunset, the delicate crescent Moon will lie just 13 degrees directly beneath Venus. The next night, Tuesday the 18th, will be even better when the Moon and Venus will be almost side by side, separated by a scant 7.6 degrees. Die-hard Star Wars fans camping out for tickets on the eve of the premier shouldn't miss this spectacle. Finally, on May 19th, the crescent Moon will climb higher in the sky, appearing approximately 20 degrees above the brilliant planet.

subscription image. This original art appears with the permission
of Bishop Web Works, copyright 1999.
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Because Venus and the Moon are so bright they should be readily visible through the glare of urban lights.

Next week's close encounter in the evening sky should be an enjoyable sight. But there's more: Cradled in the arms of the slim crescent Moon will appear the ghostly outline of the full Moon, a dim glow that astronomers call "Earthshine."

Like all the planets we see in the night sky, the Moon does not shine by its own light. It reflects sunlight. The side of the Moon facing the sun shines brightly, and the side facing away is nearly dark. The only significant illumination on the "dark side of the Moon" (no relation to the Dark Side of the Force) is due to Earthshine -- sunlight that bounces off the Earth and falls on the lunar surface. A slender crescent Moon with Earthshine is widely regarded as one of the most delicate and beautiful sights in the night sky. It will be difficult to see from urban areas, but should be easy to view from dark sky locations.

The Moon and Venus put on a dazzling show in the
Western sky  after sunset on May 18, 1999
Left: This rendering by artist Duane Hilton shows the relative positions of Venus and the Moon in the western sky on May 19, 1999. The two will be so bright that city dwellers should be able to see the sky show in spite of urban light pollution.

Most moviegoers won't have a telescope with them as they slowly approach the ticket counter, but for those who do, a look at Venus through a modest-sized instrument will be a rewarding sight. Because Venus is closer to the sun than Earth, it appears from our vantage point to have phases like the Moon. As viewed through a telescope on May 19th Venus will look like a tiny quarter Moon approximately 19 arcseconds across. Sixty percent of the visible disk will be illuminated.

If you do look at Venus through a telescope don't expect to see any surface features. The planet is completely veiled by clouds. In fact, that's why Venus is so bright. Its dense cloud layer reflects most of the sunlight shining down on the planet.

Venus is about the same size as Earth and not much closer to the sun than our own planet. For this reason it was once thought that the two planets were similar, and that advanced life might even be found on Venus. Today we know that this is not the case.

Venus as viewed by the Galileo spacecraft
Right: This colorized image of Venus was recorded by the Jupiter-bound Galileo spacecraft shortly after its gravity assist flyby of Venus in February of 1990. Galileo's glimpse of the veiled planet shows structure in swirling sulfuric acid clouds. The bright area is sunlight glinting off the upper cloud deck. more information

The pressure of Venus's atmosphere at the surface is 90 atmospheres, about the same as the pressure at a depth of 1 km in Earth's oceans. The air is consists mainly of carbon dioxide and there are several layers of clouds many kilometers thick composed of sulfuric acid. This dense atmosphere produces a runaway greenhouse effect that raises the surface temperature to over 850 o F -- hot enough to melt lead. This makes Venus's surface hotter than Mercury's, despite being nearly twice as far from the Sun.

For more information about Venus and the Moon, visit The Nine Planets by Bill Arnett and SEDS.

Web Links

Venus: Just Passing By -- Astronomy Picture of the Day, May 1, 1998

A rare double-conjunction eclipse -- Astronomy Picture of the Day, Apr. 28, 1998

A Venus Landing -- Astronomy Picture of the Day, Jan. 24, 1999

Venus's Once Molten Surface -- Astronomy Picture of the Day, Jan. 10, 1999

Why is Venus so bright? -- from

NASA's Magellan Mission to Venus -- from JPL

The Nine Planets: the Moon -- from SEDS

The Nine Planets: Venus -- from SEDS

The Solar System Photo Gallery -- from the National Space Science Data Center

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Author: Dr. Tony Phillips
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